The invention relates to a method for removing impurities from shredded plastic using a device, which comprises a first cleaning disc with a first cleaning surface and a second cleaning disc with a second cleaning surface, wherein the cleaning surfaces lie opposite each other and border a cleaning gap between each other.
Plastic refuse, for example PET beverage bottles, blister packaging made of PET (deep-drawn PET films), plastic refuse made of polyolefins or the like, must be cleaned in the course of recycling. Very high quality requirements must thereby be fulfilled. Permissible impurities lie in the ppm range. For the cleaning, plastic refuse is first milled into shredded plastic, in particular plastic flakes. Optimally milled shredded plastic is a prerequisite for the cleaning process and continuous infeed into a cleaning system. In particular, the shredded plastic should be created as evenly as possible with few fine particles. It is known to use a shredder (rotor with knives and counter knives and screen basket) for this. The generated flake size is impacted by the hole diameter in the screen basket. The separation of metals from the plastic refuse takes place in a pre-sorting by means of a magnet and eddy current separator. The state of the art is that a sorting by colour and/or plastic type takes place before the milling of the plastic refuse. However, due to the contamination of the plastic refuse, this has its limitations since the detection rate is lower for dirty materials than for clean ones. Furthermore, several washing lanes must then be operated during sorting before cleaning in order to clean the individual fractions. However, it is also possible to first pre-mill plastic refuse of different colours and/or different plastic types, then clean and to only perform the sorting by colour and/or plastic type at the end of the process by means of colour detection or respectively NIR, laser or X-ray spectroscopy. This can take place in a direct-light process and/or a transmitted-light process with suitable optical detector device (camera).
During the recycling of plastic flakes, in particular PET flakes, the following requirements must be met:    1. Removal of film and cellulose labels    2. Removal of the cellulose    3. Cleaning of the flakes of contaminants from contents (for example beverage residue)    4. Cleaning of the flakes of sticking contaminants (for example adhesives from labels)    5. Removal of metals (for example tin foil and aluminum cans)    6. Removal of foreign plastics    7. Sorting by colour (e.g. clear PET and coloured PET)
Similar requirements (but with higher threshold values) apply to the mechanical recycling of plastics. However, sorting by colour is generally omitted.
A method for the removal of cellulose and other sticking substances during recycling of refuse plastic, in particular mixed plastic, is known from WO 2008/058750 A2. A chopper disc mill can be used, the discs of which have interlocking teeth that are arranged spaced on concentric circles. Between the teeth of a circle, the holes are large enough for pieces of thicker or firmer material to pass through freely. A disc mill is also known from DE 10 2005 013 693 A1.
In the cleaning process according to the state of the art, high heating output is required to achieve sufficient cleaning of the shredded plastic. Heating outputs of 800 kW are partially required for this. This is a considerable expenditure of energy. Moreover, the control of the process temperature via a separate heating device is complex in terms of construction and feedback control.